1. Field of the Invention
The present invention is related to network analyzers in general and in particular to a full reversing millimeter test set having a band width of 75-110 GHz.
2. Description of the Prior Art
A network analyzer is used for testing and measuring the electrical characteristics of a circuit device over selected frequency bands. In practice, the input and output ports of the device, commonly called a device under test, or simply a DUT, are coupled to the analyzer by means of cables, waveguide, stripline or other suitable signal transmission lines. The tests performed typically involve first injecting a test signal into the input ports of the device and measuring the magnitude and phase angle of the signal transmitted through and reflected from the input port and thereafter injecting the test signal into the output port while measuring the magnitude and phase angle of the signal transmitted through and reflected from that port.
The results of the above-described tests are called S-parameters. When the test signal is injected into the input port of the DUT, the forward S-parameters are obtained. When the test signal is injected into the output port of the DUT, the reverse S-parameters are obtained. The forward S-parameters are defined as follows: ##EQU1## where 1 identifies the DUT input port
2 identifies the DUT output port PA1 RA=the test signal applied to the DUT input port PA1 TA=the test signal received from the DUT input port PA1 TB=the test signal received from the DUT output port. PA1 2 identifies the DUT output port PA1 RB=the test signal applied to the DUT output port PA1 TA=the test signal port received from the DUT input PA1 TB=the test signal received from the DUT output port.
The reverse S-parameters are defined as follows: ##EQU2## where 1 identifies the DUT input port
To obtain both the forward and reverse S-parameters of a DUT, the test cables must be switched between the input or output ports of the DUT or a full reversing test set must be used.
In a conventional test set using microwave test signals, i.e. signals generally having a frequency below 30 GHz, the test signals and the signals transmitted through and reflected from a DUT have typically been transmitted between the DUT and the analyzer by means of coaxial cables. It has been discovered, however, that reversing the cables or simply moving or bending the cables can produce erroneous results by introducing noise in the nature of phase distortions and the like in the signals being transmitted over the cables. This problem was recognized in U.S. Pat. No. 4,803,419 wherein the patentee proposed removing the test result signal generating means, i.e. the couplers, mixers and filters, from the analyzer proper and placing them in a test head closely coupled to the DUT.
While possibly effective at microwave signal frequencies, the patented apparatus described above makes no provision for operation at the higher millimeter wave frequencies, e.g. frequencies above 33 GHz. Signals at these frequencies are normally required to be transmitted by means of wave guide. However, the use of waveguide has been found to have several disadvantages. First, waveguide produces a loss of energy of the signal transmitted therein. Second, it is phase unstable. Third, it is bulky, cumbersome, and requires a relatively large amount of space.
In addition to requiring waveguide to transmit the test signals to and from the DUT, the conventional full reversing millimeter test set typically comprises a mechanical microwave transfer switch for selectively coupling the test signal to one of the ports while coupling a suitable termination to the other of the ports of the DUT. For example, in the typical prior known full reversing test set operable in the W band, i.e. 75-110 GHz, there is provided a four-port mechanical transfer switch comprising first and second wave guide sections which are rotatable between a first and a second position. In their first position, the first wave guide section couples a source of a test signal to the input port of the DUT while the second section couples a suitable termination to the output port of the DUT. In their second position, the first wave section couples the source of the test signal to the output of the DUT while the second section couples the termination to the input port of the DUT. The termination typically comprises a 50 ohm load.
The mechanical microwave transfer switch described above has a number of disadvantages. First, the typical transfer switch is slow in that it is motor driven and requires up to two seconds to complete a full reversal, i.e. rotation of the first and second wave guide sections between their first and second positions. Second, it is unreliable in that the switch does not park itself at a predetermined position each time it is operated. Third, it is bulky in that it typically comprises a 6-inch diameter assembly and requires a rack mounted controller with a power supply.